Spatiotemporal distribution of 137Cs in the sea surrounding Japanese Islands in the decades before the disaster at the Fukushima Daiichi Nuclear Power Plant in 2011.

The historic spatiotemporal distribution of 137Cs in the seawaters and sea-floor sediments adjacent to nuclear power plants in Japan are summarized, using data obtained over a period of time more than 20 years prior to the disaster at the Fukushima Daiichi Nuclear Power Plant in 2011. Relatively uniform distributions of 137Cs were observed both in the surface seawaters (1 m in depth) and in deeper seawaters (10 to 30 m above the seabed and ranging from tens to hundreds of meters in depth) independent of the geographical position, although lower concentrations were observed in significantly deeper bottom seawaters. Conversely, there were wide variations in 137Cs levels between sediments, such that higher 137Cs concentrations were observed in the deeper sampling locations. A mathematical model describing the successive transfer of 137Cs from surface waters through deeper waters to sediments suggested that the transfer rate of 137Cs from deep water to the sediments, and the loss rate from bottom sediments, were both greater than the transfer rate from surface water to deeper water. It was found that the calculated regression lines for 137Cs depletion rates over time for surface waters, deeper waters, and sediments were approximately parallel when plotted on a semi-logarithmic coordinate system, regardless of the sampling location. A radionuclide depletion half-life was calculated to be 4 months to 16 years with the geometric mean of 2.22 y for the sediments in the Fukushima region, suggesting that nuclear contamination will be remediated over time through sediment redistribution processes such as remobilization, bioturbation, and migration due to sea currents.